The present invention relates to a pressure sensor.
More particularly, the present invention concerns a pressure sensor comprising a vibrating member, of which the oscillation frequency and amplitude are affected by the pressure conditions of the environment where the member is located, and can therefore be exploited to measure pressure variations in the surrounding environment.
Pressure sensors, as well as, for example, temperature, density sensors, employing a vibrating member are already known. These sensors exploit the influence of the pressure, temperature and/or density conditions of the external environment on the oscillation frequency and amplitude of the vibrating member, and allow calculating the variations of the pressure, temperature or density by measuring the deviations of the oscillation frequency and amplitude from the expected values.
Sensors of the above kind are disclosed for instance in the U.S. Pat. Nos. 4,841,775, 5,152,173 and 5,426,981.
The contemporary development of the technology of microelectromechanical devices (MEMs) has allowed manufacturing miniaturised sensors, consisting of a substrate onto which a single-layer or multilayer vibrating microassembly is formed. The substrate and the vibrating microassembly are made, for instance, of silicon, silicon oxide, molybdenum, aluminium, etc.
A microelectromechanical device is illustrated by way of example in FIG. 1.
The microelectromechanical device 100 comprises a vibrating microassembly formed as a planar membrane 102 suspended above a cavity 104 formed in a supporting base 106. The supporting base 106 is preferably a silicon substrate or wafer, where cavity 104 is formed by conventional etching techniques. The membrane 102 has a substantially rectangular shape and is fastened to peripheral rim 108 surrounding cavity 104 in supporting base 106 at two rectangular fastening regions 110a, 110b adjacent to the minor sides of membrane 102. The membrane is further provided with a side extension 112 partly overlapping peripheral rim 108, so as to define a corresponding contact area 114. A metal control electrode 118 is located inside cavity 104, in contact with bottom 116 thereof, and is provided with a side extension 120 bent against side wall 122 of cavity 104. That extension partly covers peripheral rim 108 of supporting base 106 and defines a corresponding contact area 124. By applying a periodically modulated excitation voltage signal to said areas 124, 114 in control electrode 118 and membrane 102, respectively, a variable electric field can be produced between control electrode 118 and membrane 102, whereby membrane 102 is made to vibrate. Under absolute vacuum conditions, membrane 102, suitably excited, will vibrate at the resonance frequency and amplitude corresponding to vacuum conditions (intrinsic frequency and amplitude), or, if the voltage signal is a sinusoidal signal whose frequencies are different from the resonance frequency, the membrane will vibrate at the frequency imposed by said signal. When departing from the ideal condition of absolute vacuum, the presence of gas molecules or atoms in the environment surrounding the membrane will affect the frequency and amplitude of the membrane oscillations, since the free vibration of the membrane will be perturbed by the collisions with atoms and molecules. The higher the number of the atoms and molecules, hence the higher the pressure of the environment where the sensor is located, the stronger the influence. Consequently, by measuring the deviations of the frequency and/or amplitude of the vibration of membrane 102 from the expected values by means of a suitable detector, the pressure variations in the surrounding environment can be determined. Suitable materials for manufacturing membrane 102 may be aluminium, molybdenum, SiO2, Si3N4, Si (single crystalline). Moreover, membranes made of dielectric material, such as SiO2 and Si3N4, will have a sandwich structure (dielectric-metal-dielectric), with a metal layer sandwiched between two dielectric layers, so that the membrane vibration can be controlled by the electric field.
Miniaturised pressure sensors obtained by using microelectromechanical devices are disclosed, for instance, in the U.S. Pat. Nos. 5,528,939, 5,550,516 and 6,532,822.
Generally, in the prior art sensors, the microelectromechanical member is inserted into an electric circuit, and the variations of the oscillation amplitude and/or frequency of the vibrating microassembly, which relates to pressure variation, are detected from the variations of an electric parameter of the circuit.
A drawback of the prior art devices is that the measurement of the variations of the oscillation frequency and/or amplitude of the vibrating microassembly is a capacitive measurement: this prevents a direct detection of the variations, and hence, a precise and accurate measurement. According the prior art, the vibrating microassembly is placed between the plates of a capacitor belonging to the electric circuit or, possibly, the microassembly forms one of the plates of the capacitor, so that the variations in the frequency and/or amplitude of its oscillation entails capacitance variations. Yet, since measuring variations of a capacitance, which is already variable per se because of the oscillatory motion of the vibrating microassembly, would be an extremely complex, difficult to be performed in practice operation, a direct detection of the variations in the frequency and/or amplitude of the oscillatory motion is not performed. It is requested to maintain the constant value of the capacitance equal to the theoretical value, which it would have if the vibrating microassembly would oscillate with the expected frequency and amplitude values.